1. Field of the Invention
The present invention is directed to a cooling device for an X-ray source arranged at a gantry rotatable around a rotational axis, as well as to a computed tomography apparatus with such a cooling device.
2. Description of the Prior Art
Approximately 99% of the electrical energy utilized in the generation of X-rays with an X-ray source is converted into thermal energy. The heat arising in the operation of the X-ray source usually must be eliminated from the X-ray source in some manner in order to be able to operate the X-ray source over a longer time span for radiological exposures of a subject. This is particularly required when high X-ray power is needed as, for example, in computed tomography or angiography.
Added thereto as a complicating factor in computed tomography is that the X-ray source is arranged at a gantry that rotates around a rotational axis during radiological exposures. Whereas electrical energy can be supplied to the X-ray source relatively simply via wiper rings, the elimination of the heat arising during operation of the X-ray source proves problematical. X-ray sources with a rotating anode X-ray tube that are conventionally utilized in computer tomography and operate so that the heat arising during operation of the rotating anode X-ray tube is intermediately stored in the anode dish and is transferredxe2x80x94mainly by thermal radiationxe2x80x94to a coolant and insulating oil that surrounds the rotating anode X-ray tube and is contained in a housing of the X-ray radiator. The coolant and insulating oil usually circulates in a closed circulation loop through the housing of the X-ray radiator and a first heat exchanger that co-rotates with the gantry that emits the heat to the air surrounding the gantry. A second heat exchanger, that is stationary relative to the gantry, cools the heated air around the gantry and transfers the heat absorbed from the air to, for example, a stationary cooling water system.
A disadvantage of such an arrangement for cooling the rotating anode X-ray tube is that the majority of the heat transmission from the first heat exchanger to the air surrounding the gantry as well as the majority of the heat transmission from the air to the second heat exchanger takes place in a significantly locally limited manner, i.e. only to the environment of the respective locations of the X-ray source or the first heat exchanger, so that the effective area for the heat exchange is relatively small.
An object of the present invention is to implement a cooling arrangement of the type initially described wherein the elimination of heat generated during operation by an X-ray source arranged at a gantry is improved. A further object of the invention is to provide a computed tomography apparatus having an X-ray source wherein the heat generated during operation by the X-ray source arranged at a gantry is eliminated in an improved manner.
This object is inventively achieved by a cooling arrangement for an X-ray source arranged at a gantry that is rotatable around a rotational axis, the cooling arrangement having a ring-like heat exchanger that is arranged at the gantry and is connected to the X-ray source in thermally conductive fashion. The ring-like fashioning of the heat exchanger, i.e. the fashioning thereof adapted to the preferred form of the gantry, enables a large-area heat transfer of the heat arising when generating X-rays with the X-ray source from the heat exchanger to the air surrounding the heat exchanger in a simple way, with heat transfer occurring over the entire circumferential surface of the ring-like heat exchanger. Although the heat transfer from the X-ray source to the heat exchanger is still relatively locally limited, the area for the heat transfer from the heat exchanger to the air surrounding the heat exchanger is enlarged so that the elimination of the heat is significantly improved. Whereas the area for the heat exchange for current, standard heat exchangers provided at the gantry amounts to approximately 0.1 m2, three times the area for the heat exchanger is already obtained with the inventive ring-like, heat exchanger adapted to the size of the gantry that has a radius of approximately 0.5 m and a width of approximately 0.1 m. The heat exchanger is preferably arranged ring-like around the gantry, however, the heat exchanger alternatively can be arranged axially offset relative to the gantry in the direction of the rotational axis and preferably with substantially the same radius as the gantry.
In a preferred embodiment of the invention the heat exchanger is rotatable around the rotational axis together with the gantry. The heat transfer from the X-ray source to the first heat exchanger therefore can be accomplished in a simple way by thermal conduction since no parts of the gantry and the heat exchanger that move relative to one another and form heat barriers are present.
In a version of the invention the heat exchanger is formed of at least one heat exchange element, or at least two heat exchange elements that are arranged annularly and are connected to one another and to the X-ray source in thermally conductive fashion. This version thus allows the use of individual heat exchange elements that are commercially obtainable and thus economical, of the type, for example, employed in the automotive industry. When arranged ring-like and connected to one another, these heat exchange elements, which are usually cuboid, result in a heat exchanger which effectively eliminates the heat generated during operation of the X-ray source for the X-ray source to the heat exchanger.
A more uniform distribution of the heat generated by the X-ray source and absorbed by the heat exchanger is achieved in a version of the invention wherein a medium flows through the heat exchanger in a closed circulation loop. The medium preferably flows not only through the heat exchange elements connected to one another by, for example, conduits but also flows through cooling and insulating oil that surrounds the X-ray source. This results in a relatively uniform distribution of the heat over the heat exchanger, and thus a large-area heat transfer from the heat exchanger to the air surrounding the heat exchanger. For example, the cooling and insulating oil also can be employed as the flowing medium.
In another version of the invention coverings extending between the heat exchange elements in the circumferential direction connect the heat exchange elements to one another, and/or annular guide devices at both sides of the heat exchanger are provided. These coverings and/or guide elements conduct an air stream produced by rotation of the heat exchanger, and heated at the heat exchanger, toward the exterior away from the gantry. In this way, heated air is prevented from proceeding into the interior of the gantry which could negatively influence the functioning of the radiation detector (usually arranged at the gantry) and its appertaining electronics.
In another preferred embodiment of the invention, the aforementioned heat exchanger serves as first heat exchanger, and a second heat exchanger is provided that interacts with the first heat exchanger, resulting in the elimination of the heat generated by the X-ray source being further improved. In a version of this embodiment, the second heat exchanger is stationary relative to the first heat exchanger, and the second heat exchanger is either annularly arranged around the first heat exchanger or is attached to the first heat exchanger axially offset in the direction of the rotational axis. In all instances, the two heat exchangers preferably reside extremely closely opposite one another, separated only by an air gap, so thatxe2x80x94due to this arrangementxe2x80x94the second heat exchanger is charged with only a comparatively slight, secondary stream of air that has not been heated at the first heat exchanger. As a result, a higher temperature difference between the primary side and the secondary side (i.e. the side absorbing the heat and the side emitting the heat, for example to the air surrounding the second heat exchanger), that is directly, proportional to the cooling power is effective at the second heat exchanger.
In another embodiment of the invention, the second heat exchanger also can be formed of one or more heat exchange elements connected to one another, or a medium can flow through the second heat exchanger. Commercially obtainable heat exchange elements also can be employed to form the second heat exchanger to the lower the manufacturing costs. The connection of the heat exchange elements to one another and having a medium flow through the heat exchange elements produce an optimally uniform distribution of the heat over the second heat exchangerxe2x80x94as in the case of the first heat exchangerxe2x80x94and thus resulting in a large-area heat transfer from the second heat exchanger to, for example the air surrounding the second heat exchanger.
In another version of the invention, the second heat exchanger also has coverings extending between the heat exchange elements thereof in the circumferential direction that connect the heat exchange elements to one another. The first and second heat exchangers can have inter-engaging, annular guide devices that guide an air stream generated as a result of the rotation of the first heat exchanger, and heated at the first heat exchanger, to the second heat exchanger. The coverings between the heat exchange elements of the second heat exchanger prevent penetration of the non-heated air into the air gap between the first and second heat exchangers. Additionally, the inter-engaging, annular guide devices cause the air heated at the first heat exchanger to be guided in the direction of the second heat exchanger so that this heated air cannot undesirably proceed into the inside of the gantry and negatively influence the functioning of the radiation detector and its appertaining electronics.